(a) Field
The present invention relates to a display device. More specifically, the present invention relates to a method for improving an aperture ratio of an organic EL (electroluminescent) display device.
(b) Description of the Related Art
In general, an organic EL display electrically excites a phosphorous organic compound to emit light, and it voltage- or current-drives N×M organic emitting cells to display images. An organic emitting cell includes an anode, such as indium tin oxide (ITO), an organic thin film, and a cathode layer (metal). The organic thin film has a multi-layer structure including an emitting layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL) for maintaining balance between electrons and holes and improving emitting efficiencies. It further includes an electron injecting layer (EIL) and a hole injecting layer (HIL).
Methods for driving the organic emitting cells include a passive matrix method, and an active matrix method using thin film transistors (TFTs). In the passive matrix method, cathodes and anodes cross (i.e., cross over or intersect with) each other, and lines are selectively driven. On the other hand, in the active matrix method, a TFT is coupled to each ITO pixel electrode to thereby maintain the voltage by capacitance of a capacitor. The active matrix method is classified as a voltage programming method or a current programming method according to signal forms supplied for programming a voltage in the capacitor.
It is difficult for the conventional voltage-programming pixel circuit to obtain high gray scales due to the threshold voltage (VTH) of a TFT and the deviation of the mobility of carriers caused by non-uniformity of the manufacturing process. For example, when a TFT is driven by a voltage of 3 volts (3V), the voltage is applied to a gate of the TFT at intervals of less than 12 mV (=3V/256) in order to represent 8-bit (256) gray scales. Therefore, for example, if the deviation of the threshold voltage of the TFT is 100 mV because of non-uniformity of a manufacturing process, it becomes difficult to represent the high gray scales.
In order to compensate for the deviation of the threshold voltage of the TFT, a diode-connected compensation transistor is conventionally coupled to the gate of a driving transistor.
The diode-connected transistor represents a transistor which substantially performs the same operation as that of the diode, and indicates a transistor having a gate and a drain which are coupled to each other as shown in FIGS. 1A and 1B.
FIG. 2 shows a plane view of a conventional diode-connected transistor, and FIG. 3 shows a cross-sectional view of FIG. 2 with respect to the reference of A-B.
As shown in FIGS. 2 and 3, a conventional diode-connected transistor includes a passivation layer 80, a drain electrode 32 contacting a drain region 31, a source electrode 22 contacting a source region 21, and a gate electrode 10. The drain electrode 32 is extended to reach the gate electrode 10, and the gate electrode 10 and the drain electrode 32 are coupled through a contact hole 72 in a second insulation film 70. This method reduces the aperture ratio of the organic EL display device since the area occupied by the diode-connected transistor is enlarged.